Methods of forming field effect transistors and field effect transistor circuitry

ABSTRACT

Methods of forming field effect transistors and resultant field effect transistor circuitry are described. In one embodiment, a semiconductive substrate includes a field effect transistor having a body. A first resistive element is received by the substrate and connected between the transistor&#39;s gate and the body. A second resistive element is received by the substrate and connected between the body and a reference voltage node. The first and second resistive elements form a voltage divider which is configured to selectively change threshold voltages of the field effect transistor with state changes in the gate voltage. In a preferred embodiment, first and second diode assemblies are positioned over the substrate and connected between the gate and body, and the body and a reference voltage node to provide the voltage divider.

TECHNICAL FIELD

This invention relates to methods of forming field effect transistors,and to field effect transistor circuitry.

BACKGROUND OF THE INVENTION

It is desirable in transistors to be able to drive high currents.Driving high currents can enhance a transistor's operating performanceincluding its operating speed. In field effect transistors (FETs),current ii flow is primarily conducted by way of the drain-to-sourcecurrent I_(ds). While higher drive currents can be achieved by buildingwider FET devices, tradeoffs are made in valuable wafer real estate.Larger devices also typically have larger capacitances which canadversely impact device performance. Also typically, a high I_(ds)current in FET devices can result in an increased sub-threshold currentleakage. It is desirable in FETs to minimize the sub-threshold currentleakage. Accordingly, it is desirable to have the I_(ds) ratio ofon-state current (I_(on)) to off-state current (I_(off)) be as high aspossible. Such improves sub-threshold device leakage characteristics aswell as increases the transistor's operating speed performance.

This invention arose out of concerns associated with improving fieldeffect transistor performance.

SUMMARY OF THE INVENTION

Methods of forming field effect transistors and resultant field effecttransistor circuitry are described. In one embodiment, a semiconductivesubstrate includes a field effect transistor having a body. A firstresistive element is received by the substrate and connected between thetransistor's gate and the body. A second resistive element is receivedby the substrate and connected between the body and a reference voltagenode. The first and second resistive elements form a voltage dividerwhich is configured to selectively change threshold voltages of thefield effect transistor with state changes in the gate voltage. In apreferred embodiment, first and second diode assemblies are positionedover the substrate and connected between the gate and body, and the bodyand a reference voltage node to provide the voltage divider.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a top plan view of a semiconductor wafer fragment in processin accordance with one embodiment of the invention.

FIG. 2 is a diagrammatic side sectional view of the FIG. 1 waferfragment taken along line 2—2 in FIG. 1.

FIG. 3 is a view of the FIG. 2 wafer fragment at a processing stepsubsequent to that shown in FIG. 2.

FIG. 4 is a top plan view of the FIG. 1 wafer fragment at a processingstep subsequent to that shown in FIG. 1.

FIG. 5 is a view which is taken along line 5—5 in FIG. 4.

FIG. 6 is a view of the FIG. 5 wafer fragment at a processing stepsubsequent to that shown in FIG. 5.

FIG. 7 is a top plan view of the FIG. 4 wafer fragment at a processingstep subsequent to that shown in FIG. 4.

FIG. 8 is a view of the FIG. 7 wafer fragment taken along line 8—8 inFIG. 7.

FIG. 9 is a view of the FIG. 7 wafer fragment at a processing stepsubsequent to that shown in FIG. 7.

FIG. 10 is a view of the FIG. 9 wafer fragment taken along line 10—10 inFIG. 9.

FIG. 11 is a view of the FIG. 9 wafer fragment at a processing stepwhich is subsequent to that shown in FIG. 9.

FIG. 12 is a view which is taken along line 12—12 in FIG. 11.

FIG. 13 is a view of the FIG. 11 wafer fragment at a processing stepsubsequent to that shown in FIG. 11.

FIG. 14 is a view of the FIG. 13 wafer fragment at a processing stepsubsequent to that shown in FIG. 13.

FIG. 15 is a view which is taken along line 15—15 in FIG. 14.

FIG. 16 is a view of the FIG. 14 wafer fragment at a processing stepsubsequent to that shown in FIG. 14.

FIG. 17 is a schematic diagram of field effect transistor circuitryconstructed in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts”(Article 1, Section 8).

Referring to FIGS. 1 and 2, a semiconductor wafer fragment 20 includes asemiconductive substrate 22 having a substrate region 23. Region 23comprises a body of a field effect transistor which is to be formed. Inthe illustrated example, substrate 22 includes a buried mega eV (MeV)layer 24 and a pair of isolation regions 26 which collectively isolatethe body. Where n-channel devices are to be formed, substrate region 23comprises p-type material and layer 24 comprises n-type material. Wherep-channel devices are to be formed, substrate region 23 comprises n-typematerial and layer 24 comprises p-type material. Layer 24 and isolationregions 26 define an area 28 over or within which field effecttransistor circuitry is to be formed. The substrate can comprise anysuitable substrate. In the context of this document, the term“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. The substrate can also comprise silicon-on-insulator substratesformed through various known techniques.

Referring to FIG. 3, a gate oxide layer 30 is formed over substrate 22.

Referring to FIGS. 4 and 5, a first patterned masking layer 32 is formedover substrate 22 and defines an opening 34 over area 28. Layer 32 cancomprise photoresist. Dopant is introduced into the substrate throughopening 34 to form a buried contact diffusion region 36. In theillustrated example, layer 24 comprises an n-type material, substrateregion 23 comprises a p-type material, and buried contact diffusionregion 36 comprises a p+ diffusion region. A wet oxide etch can beconducted either prior to or subsequent to formation of diffusion region36 to remove portions of oxide layer 30 from over the substrate areawhere dopant is to be introduced.

Referring to FIG. 6, first patterned masking layer 32 is removed.

Referring to FIGS. 7 and 8, a conductive material layer 38 is formedover substrate 22 and in electrical communication with buried contactdiffusion region 36. In a preferred embodiment, layer 38 is in physicalcontact with diffusion region 36. An exemplary material is in situn-type doped polysilicon.

Referring to FIGS. 9 and 10, a patterned masking layer 40 is formed oversubstrate 22 and defines a conductive line pattern. Layer 40 cancomprise photoresist.

Referring to FIGS. 11 and 12, conductive layer 38 is etched to provide aconductive line 42 which is in electrical communication with buriedcontact diffusion region 36 (FIG. 12). Conductive line 42 includes aportion 44 which provides a gate for a field effect transistor which isto be formed.

Referring to FIG. 13, a patterned masking layer 46 is formed oversubstrate 22 and includes masking openings 48, 50 and 52 over selectedportions of conductive line 42. Exposed conductive line portions arefirst doped with a first-type dopant which, in the preferred embodiment,comprises p-type dopant. In one embodiment, CMOS circuitry can becontemporaneously formed with the present field effect transistor. Inthis case, the p-type doping of the conductive line portions can alsodope other portions of the wafer where, for example, p-channel devicesare being formed. The p-type regions formed within conductive line 42are spaced-apart along and within the line. Layer 46 is subsequentlyremoved.

In this example, the material comprising conductive line 42 is in situn-type doped polysilicon. Accordingly, the doping of the selectedportions of conductive line 42 just described provides alternatingregions of p-type dopant and n-type dopant within conductive line 42.Accordingly, a plurality of pn junctions 66 (FIG. 16) are distributedthroughout conductive line 42. In the event that the material comprisingconductive line 42 is not in situ doped n-type polysilicon, or in theevent conductive line 42 is desired to be rendered even more n-type, itcan be further doped with n-type impurity described just below.

Referring to FIG. 14, a patterned masking layer 54 is formed oversubstrate 22 and has masking openings over the illustrated cross-hatchedportions of conductive line 42 and other transistor structures whichdefine substrate areas within which source/drain regions are to beformed. Exposed conductive line portions are second doped with asecond-type dopant which is different from the first-type dopant.Preferably, the second-type dopant is n-type dopant. Doping also occurssufficiently to form source drain regions 62, 64 (FIG. 15) withinsubstrate 22. Desired spacing between diffusion regions 64 and 36, andthe dopant concentration of region 23 therebetween, can combine to avoidZener diode action.

The doping of the selected portions of conductive line 42 just describedprovides alternating regions of p-type dopant and n-type dopant withinconductive line 42. Accordingly, the dopings define a plurality of pnjunctions 66 (FIG. 16) which are distributed throughout conductive line42.

In one embodiment, that portion of conductive line 42 comprising thegate line of the transistor can subjected to silicide processing. Inthis embodiment, spacers can be provided over the line, and a protectivelayer such as an oxide formed through decomposition of TEOS can beformed over the substrate and subsequently patterned to overlie regions72-80. The protective layer can also overlie other portions of thesubstrate which are not to be subjected to silicide processing. Arefractory metal such as titanium can be formed over the subjectpolysilicon, and subsequently heat processed to form a silicide layerover the substrate. The silicide can also be formed over thesource/drain regions depending upon design considerations.

Referring to FIG. 16, two nodes 68, 70 are provided. The nodes areprovided by forming openings (designated at “x”) through an insulativematerial (not shown), and subsequently filling the openings withconductive material. Metal lines are provided over the insulativematerial and comprise node portions which are designated with the leadlines of designators 68, 70 respectively. Node 68 provides a referencevoltage node which extends to a reference voltage. Node 70 provides agate voltage node.

Referring to FIGS. 16 and 17, a plurality of regions 72, 74, 76, 78 and80 are formed within conductive line 42 and are arranged to provide avoltage divider circuit which is connected between gate 44, body 23, andreference voltage node 68. The voltage divider circuit is preferablyconfigured to selectively change threshold voltages of the field effecttransistor with state changes in the gate voltage as will becomeapparent below.

In one embodiment, regions 72-80 are arranged to provide first andsecond resistive elements received by, supported by, or preferablypositioned over the substrate and suitably connected to provide thevoltage divider circuit. In this example, a first resistive elementincludes regions 72, 74 and 76 (a first p-n-p structure), and a secondresistive element includes regions 76, 78 and 80 (a second p-n-pstructure).

In another embodiment, the plurality of regions 72-80 are arrangedwithin conductive line 42 to provide first and second diode assemblies82, 84 (FIG. 17). The diode assemblies are received by, supported by, orpreferably positioned over the substrate, with the first diode assembly82 connected between gate 44 and body 23, and second diode assembly 84connected between body 23 and reference voltage node 68.

In a preferred embodiment, first diode assembly 82 comprises a pair ofdiodes D₁, D₂. Each diode has an anode and a cathode, with the cathodesbeing connected together in a cathode-connected configuration as shown.The anode of diode D₁ is connected with gate 44 while the anode of diodeD₂ is connected with body 23. Diode assembly 84 preferably comprises apair of diodes D₃, D₄, each diode having an anode and a cathode with thecathodes being connected together in a cathode-connected configurationas shown. The anode of diode D₃ is connected to body 23, and the anodeof D₄ is connected to reference voltage node 68.

In operation, the field effect transistor of the present inventionprovides monolithic transistor construction with a bi-level thresholdvoltage which increases the ratio of I_(on) to I_(off). The transistor'sgate is operably connected with the substrate and is configured tochange the potential of the substrate. Accordingly, the transistor isselectively configurable by the substrate potential to have differentthreshold voltages. As an example, with the drain current being in thepico ampere range, when the gate voltage V_(G) is high (e.g. 3.3 volts),the bulk or body is at 0 volts and the threshold voltage is adjusted bythe enhancement implant to be between 0.5 volts to 0 volts. This isdesirable for an aggressive I_(DS) conduction. When the gate voltageV_(G) is low (e.g. 0 volts), the bulk or body is at −1.5 volts and thethreshold voltage moves up to 1.0 volts according to the doping profilein the bulk of the n-channel device. In this example, the referencevoltage V_(ref) is −3.0 volts. The gate voltage can be less than 0 voltsdepending on the gate-induced drain leakage of the device as well.

Field-effect transistors constructed in accordance with the inventioncan have enhanced speed performance which justifies any increase incircuit layout. Other advantages of the invention can include a fairlysimplified process flow which enhances the manufacturability of thedevice. The field effect transistor can have improved drive, as well asan improved on/off current ratio. Accordingly, faster devices can beimplemented in current CMOS technologies without sacrificing sub24threshold and/or stand-by current.

While the inventive methods and structures have been described in thecontext of n-channel devices, it is to be understood that the inventivemethods and structures can be equally well-suited for application withp-channel devices, with consideration being given for the inherentdifferences between n- and p-channel devices.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A method of forming a field effect transistorcomprising: providing a semiconductive substrate; forming a field effecttransistor having a p-type body within the substrate, the transistorhaving a gate, the p-type body including a p+ contact thereto; forming avoltage divider circuit over the substrate and connected with the gate,the p-type body, and a reference voltage node, forming the voltagedivider circuit comprising: forming a first diode having a first anodecoupled to the gate and a first cathode; forming a second diode having asecond cathode coupled to the first cathode and a second anode coupledto the p+ contact; forming a third diode having a third anode coupled tothe p+ contact and a third cathode; and forming a fourth diode having afourth anode coupled to the reference voltage node and a fourth cathodecoupled to the third cathode, the voltage divider circuit beingconfigured to selectively change a threshold voltage of the field effecttransistor when a gate voltage of the field effect transistor changesfrom high to low, and vice versa.
 2. The method of claim 1, whereinforming the voltage divider circuit comprises forming the voltagedivider circuit to be configured to change the threshold voltage frombetween one-half of a volt and zero volts when a gate voltage of thefield effect transistor is high to another threshold voltage of aboutone volt when the gate voltage is low.
 3. The method of claim 1, whereinforming the first, second, third and fourth diodes comprises: forming alayer of polysilicon over the substrate; forming a conductive line fromthe conductive layer of polysilicon, a portion of the conductive linecomprising the gate; doping selected portions of the conductive linewith n-type dopant to form the first, second, third and fourth cathodes;and doping other selected portions of the conductive line with p-typedopant to form the first, second, third and fourth anodes.
 4. The methodof claim 1, wherein forming a field effect transistor comprises dopinginto the semiconductor substrate through a patterned masking layer toform a p+ doped buried contact in a p-type substrate prior to formingthe voltage divider circuit.
 5. The method of claim 1, wherein formingthe voltage divider circuit comprises forming the voltage dividercircuit to be configured to change a voltage of the body from about zerovolts when the gate voltage is high, to about minus one point five voltswhen the gate voltage is low.
 6. The method of claim 1, wherein formingthe voltage divider circuit comprises forming the voltage dividercircuit to be configured to change the threshold voltage from betweenone-half volt and zero volts and to change the voltage of the body fromat about zero volts when a gate voltage of the field effect transistoris high, to a new threshold voltage of about one volt and a new voltageof the body of about minus one point five volts when the gate voltage islow.
 7. A method of forming a field effect transistor comprising:providing a semiconductive substrate; forming a field effect transistorhaving a p-type body within the substrate, the transistor having a gate,the p-type body including a p+ contact thereto; forming a voltagedivider circuit over the substrate and connected with the gate, the p+contact to the body, and a reference voltage node, the voltage dividercircuit being configured to selectively change a threshold voltage ofthe field effect transistor when a gate voltage of the field effecttransistor changes from high to low, and vice versa, forming the voltagedivider circuit comprising: forming a conductive layer of material overthe substrate; etching the conductive layer of material to provide aconductive line, at least a portion of the conductive line providing thegate for the transistor, the conductive line extending to the voltagereference node; and doping selected portions of the conductive line toform first and second resistive elements, the first resistive elementbeing coupled between the gate and the p+ contact to the body and thesecond resistive element being coupled between the p+ contact to thebody and the reference voltage node.
 8. The method of claim 7, whereindoping selected portions of the conductive line to form first and secondresistive elements includes forming the first resistive elements by:forming a first diode having a first anode coupled to the gate and afirst cathode; and forming a second diode having a second cathodecoupled to the first anode and a second anode coupled to the p+ contact.9. The method of claim 8, wherein doping selected portions of theconductive line to form first and second resistive elements includesforming the second resistive elements by: forming a third diode having athird anode coupled to the p+ contact and a third cathode; and forming afourth diode having a fourth anode coupled to the reference voltage nodeand a fourth cathode coupled to the third cathode, the voltage dividercircuit being configured to selectively change the threshold voltage ofthe field effect transistor when the gate voltage of the field effecttransistor changes from high to low, and vice versa.